1. Field of the Invention
This disclosure relates to a power-supply device using a transformer and an image formation apparatus including the power-supply device, and more particularly relates to a power-supply device used in electrophotographic equipment and an image formation apparatus including the power-supply device.
2. Description of Related Art
A high-voltage power supply using a transformer, such as an electromagnetic transformer or a piezoelectric transformer, is widely used in an electrophotographic image formation apparatus. An electrophotographic image formation process includes a series of steps including: a charge step of uniformly charging a surface of a photoconductor; an exposure step of forming an electrostatic latent image by irradiating the surface of the photoconductor with light; a development step of forming a developer image on the photoconductor by allowing a charged developer to adhere to the electrostatic latent image; a transfer step of transferring the developer image onto a recording medium such as paper; and a fixing step of fixing the transferred developer image to the recording medium. The high-voltage power supply can supply a bias voltage to the members (loads) used in such an image formation process.
For example, Patent Document 1 (Japanese Patent Application Publication No. 2006-201351 (e.g., FIG. 1 and Paragraphs [0046] to [0055] and FIG. 14 and Paragraphs [0083] to [0098])) discloses a high-voltage power supply using a piezoelectric transformer. The high-voltage power supply disclosed in Patent Document 1 supplies a load, such as a transfer roller, with an output voltage or an output current corresponding to an inputted target voltage value. The high-voltage power supply has a protection function to detect an overcurrent or overvoltage and lower the target voltage value when the overcurrent or overvoltage is supplied to the load due to a change in electrical characteristics of the load. When the protection function operates to lower the target voltage value, the output voltage or the output current is lowered.